Among the so-far known processes for producing an optically active .beta.-amino-.alpha.-hydroxycarboxylic acid derivative, there may be mentioned, for instance, the process comprising cyanizing an N-protected phenyl-alanial derivative and then hydrolyzing the resultant derivative ((1) Synthesis, 1989, page 709; (2) Journal of Medicinal Chemistry, vol. 37, page 2918, 1994; (3) Journal of Medicinal Chemistry, vol. 20, page 510, 1977).
However, the process comprising cyanizing an N-protected phenylalaninal derivative and then hydrolyzing the resultant derivative is not suited for the production of an erythro-form .beta.-amino-.alpha.-hydroxy acid derivative since the stereoselectivity is of the so-called threo-selective type or almost no stereoselectivity is found. It has a problem in that the use of a strongly toxic cyanizing agent is required. Said threo configuration indicates a relative configuration opposite to the erythro configuration mentioned above.
A process is also known which comprises stereoselectively adding N-benzyl-.alpha.-phenethylamine to .alpha., .beta.-unsaturated esters in the manner of Michael addition, followed by hydroxylation (Synlett, vol. 10, page 731, 1993), for instance. However, it has a problem in that not less than equivalent amounts of the optically active amine and oxidizing agent have to be used.